1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to press fabrics for the press section of a paper machine.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
The present invention relates specifically to the press fabrics used in the press section. Press fabrics play a critical role during the paper manufacturing process. One of their functions, as implied above, is to support and to carry the paper product being manufactured through the press nips.
Press fabrics also participate in the finishing of the surface of the paper sheet. That is, press fabrics are designed to have smooth surfaces and uniformly resilient structures, so that, in the course of passing through the press nips, a smooth, mark-free surface is imparted to the paper.
Perhaps most importantly, the press fabrics accept the large quantities of water extracted from the wet paper in the press nip. In order to fill this function, there literally must be space, commonly referred to as void volume, within the press fabric for the water to go, and the fabric must have adequate permeability to water for its entire useful life. Finally, press fabrics must be able to prevent the water accepted from the wet paper from returning to and rewetting the paper upon exit from the press nip.
Contemporary press fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a woven base fabric into which has been needled a batt of fine, non-woven fibrous material. The base fabrics may be woven from monofilament, plied monofilament, multifilament or plied multifilament yarns, and may be single-layered, multi-layered or laminated. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
The woven base fabrics themselves take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a woven seam. Alternatively, they may be produced by a process commonly known as modified endless weaving, wherein the widthwise edges of the base fabric are provided with seaming loops using the machine-direction (MD) yarns thereof. In this process, the MD yarns weave continuously back and forth between the widthwise edges of the fabric, at each edge turning back and forming a seaming loop. A base fabric produced in this fashion is placed into endless form during installation on a paper machine, and for this reason is referred to as an on-machine-seamable fabric. To place such a fabric into endless form, the two widthwise edges are brought together, the seaming loops at the two edges are interdigitated with one another, and a seaming pin or pintle is directed through the passage formed by the interdigitated seaming loops.
Further, the woven base fabrics may be laminated by placing one base fabric within the endless loop formed by another, and by needling a staple fiber batt through both base fabrics to join them to one another. One or both woven base fabrics may be of the on-machine-seamable type.
In any event, the woven base fabrics are in the form of endless loops, or are seamable into such forms, having a specific length, measured longitudinally therearound, and a specific width, measured transversely thereacross. Because paper machine configurations vary widely, paper machine clothing manufacturers are required to produce press fabrics, and other paper machine clothing, to the dimensions required to fit particular positions in the paper machines of their customers. Needless to say, this requirement makes it difficult to streamline the manufacturing process, as each press fabric must typically be made to order.
In response to this need to produce press fabrics in a variety of lengths and widths more quickly and efficiently, press fabrics have been produced in recent years using a spiral technique disclosed in commonly assigned U.S. Pat. No. 5,360,656 to Rexfelt et al., the teachings of which are incorporated herein by reference.
U.S. Pat. No. 5,360,656 shows a press fabric comprising a base fabric having one or more layers of staple fiber material needled thereinto. The base fabric comprises at least one layer composed of a spirally wound strip of woven fabric having a width which is smaller than the width of the base fabric. The base fabric is endless in the longitudinal, or machine, direction. Lengthwise threads of the spirally wound strip make an angle with the longitudinal direction of the press fabric. The strip of woven fabric may be flat-woven on a loom which is narrower than those typically used in the production of paper machine clothing.
The base fabric comprises a plurality of spirally wound and joined turns of the relatively narrow woven fabric strip. The fabric strip is woven from lengthwise (warp) and crosswise (filling) yarns. Adjacent turns of the spirally wound fabric strip may be abutted against one another, and the helically continuous seam so produced may be closed by sewing, stitching, melting, welding (e.g. ultrasonic) or gluing. Alternatively, adjacent longitudinal edge portions of adjoining spiral turns may be arranged overlappingly, so long as the edges have a reduced thickness, so as not to give rise to an increased thickness in the area of the overlap. Further, the spacing between lengthwise yarns may be increased at the edges of the strip, so that, when adjoining spiral turns are arranged overlappingly, there may be an unchanged spacing between lengthwise threads in the area of the overlap.
In any case, a woven base fabric, taking the form of an endless loop and having an inner surface, a longitudinal (machine) direction and a transverse (cross-machine) direction, is the result. The lateral edges of the woven base fabric are then trimmed to render them parallel to its longitudinal (machine) direction. The angle between the machine direction of the woven base fabric and the helically continuous seam may be relatively small, that is, typically less than 10xc2x0. By the same token, the lengthwise (warp) yarns of the woven fabric strip make the same relatively small angle with the longitudinal (machine) direction of the woven base fabric. Similarly, the crosswise (filling) yarns of the woven fabric strip, being perpendicular to the lengthwise (warp) yarns, make the same relatively small angle with the transverse (cross-machine) direction of the woven base fabric. In short, neither the lengthwise (warp) nor the crosswise (filling) yarns of the woven fabric strip align with the longitudinal (machine) or transverse (cross-machine) directions of the woven base fabric.
In the method shown in U.S. Pat. No. 5,360,656, the woven fabric strip is wound around two parallel rolls to assemble the woven base fabric. It will be recognized that endless base fabrics in a variety of lengths and widths may be provided by spirally winding a relatively narrow piece of woven fabric strip around the two parallel rolls, the length of a particular endless base fabric being determined by the length of each spiral turn of the woven fabric strip, and the width being determined by the number of spiral turns of the woven fabric strip. The prior necessity of weaving complete base fabrics of specified lengths and widths to order may thereby be avoided. Instead, a loom as narrow as 20 inches (0.5 meters) could be used to produce a woven fabric strip, but, for reasons of practicality, a conventional textile loom having a width of from 40 to 60 inches (1.0 to 1.5 meters) may be preferred.
U.S. Pat. No. 5,360,656 also shows a press fabric comprising a base fabric having two layers, each composed of a spirally wound strip of woven fabric. Both layers take the form of an endless loop, one being inside the endless loop formed by the other. Preferably, the spirally wound strip of woven fabric in one layer spirals in a direction opposite to that of the strip of woven fabric in the other layer. That is to say, more specifically, the spirally wound strip in one layer defines a right-handed spiral, while that in the other layer defines a left-handed spiral. In such a two-layer, laminated base fabric, the lengthwise (warp) yarns of the woven fabric strip in each of the two layers make relatively small angles with the longitudinal (machine) direction of the woven base fabric, and the lengthwise (warp) yarns of the woven fabric strip in one layer make an angle with the lengthwise (warp) yarns of the woven fabric strip in the other layer. Similarly, the crosswise (filling) yarns of the woven fabric strip in each of the two layers make relatively small angles with the transverse (cross-machine) direction of the woven base fabric, and the crosswise (filling) yarns of the woven fabric strip in one layer make an angle with the crosswise (filling) yarns of the woven fabric strip in the other layer. In short, neither the lengthwise (warp) nor the crosswise (filling) yarns of the woven fabric strip in either layer align with the longitudinal (machine) or transverse (cross-machine) directions of the base fabric. Further, neither the lengthwise (warp) nor the crosswise (filling) yarns of the woven fabric strip in either layer align with those of the other.
As a consequence, the base fabrics shown in U.S. Pat. No. 5,360,656 have no defined machine- or cross-machine-direction yarns. Instead, the yarn systems lie in directions at oblique angles to the machine and cross-machine directions. A press fabric having such a base fabric may be referred to as a multiaxial press fabric. Whereas the standard press fabrics of the prior art have three axes: one in the machine direction (MD), one in the cross-machine direction (CD), and one in the z-direction, which is through the thickness of the fabric, a multiaxial press fabric has not only these three axes, but also has at least two more axes defined by the directions of the yarn systems in its spirally wound layer or layers. Moreover, there are multiple flow paths in the z-direction of a multiaxial press fabric. As a consequence, a multiaxial press fabric has at least five axes. Because of its multiaxial structure, a multiaxial press fabric having more than one layer exhibits superior resistance to nesting and/or to collapse in response to compression in a press nip during the papermaking process as compared to one having base fabric layers whose yarn systems are parallel to one another.
Until recently, multiaxial press fabrics of the foregoing type had been produced only in endless form. As such, their use had been limited to press sections having cantilevered press rolls and other components, which permit an endless press fabric to be installed from the side of the press section. However, their relative ease of manufacture and superior resistance to compaction contributed to an increased interest and a growing need for a multiaxial press fabric which could be seamed into endless form during installation on a press section, thereby making such press fabric available for use on paper machines lacking cantilevered components. On-machine-seamable multiaxial press fabrics, developed to meet this need, are shown in commonly assigned U.S. Pat. Nos. 5,916,421; 5,939,176; and 6,117,274 to Yook, the teachings of which are incorporated herein by reference.
U.S. Pat. No. 5,916,421 shows an on-machine-seamable multiaxial press fabric for the press section of a paper machine made from a base fabric layer assembled by spirally winding a fabric strip in a plurality of contiguous turns, each of which abuts against and is attached to those adjacent thereto. The resulting endless base fabric layer is flattened to produce first and second plies joined to one another at folds at their widthwise edges. Crosswise yarns are removed from each turn of the fabric strip at folds at the widthwise edges to produce unbound sections of lengthwise yarns. A seaming element, having seaming loops along one of its widthwise edges, is disposed between the first and second fabric plies at each of the folds at the two widthwise edges of the flattened base fabric layer. The seaming loops extend outwardly between the unbound sections of the lengthwise yarns from between the first and second fabric plies. The first and second fabric plies are laminated to one another by needling staple fiber batt material therethrough. The press fabric is joined into endless form during installation on a paper machine by directing a pintle through the passage formed by the interdigitation of the seaming loops at the two widthwise edges.
U.S. Pat. No. 5,939,176 also shows an on-machine-seamable multiaxial press fabric. Again, the press fabric is made from a base fabric layer assembled by spirally winding a fabric strip in a plurality of contiguous turns, each of which abuts against and is attached to those adjacent thereto. The resulting endless fabric layer is flattened to produce a first and second fabric plies joined to one another at folds at their widthwise edges. Crosswise yarns are removed from each turn of the fabric strip at the folds at the widthwise edges to produce seaming loops. The first and second plies are laminated to one another by needling staple fiber batt material therethrough. The press fabric is joined into endless form during installation on a paper machine by directing a pintle through the passage formed by the interdigitation of the seaming loops at the two widthwise edges.
Finally, in U.S. Pat. No. 6,117,274, another on-machine-seamable multiaxial press fabric is shown. Again, the press fabric is made from a base fabric layer assembled by spirally winding a fabric strip in a plurality of contiguous turns, each of which abuts against and is attached to those adjacent thereto. The resulting endless fabric layer is flattened to produce a first and second fabric plies joined to one another at folds at their widthwise edges. Crosswise yarns are removed from each turn of the fabric strip at the folds at the widthwise edges to produce unbound sections of lengthwise yarns. Subsequently, an on-machine-seamable base fabric, having seaming loops along its widthwise edges, is disposed between the first and second fabric plies of the flattened base fabric layer. The seaming loops extend outwardly between the unbound sections of the lengthwise yarns from between the first and second fabric plies. The first fabric ply, the on-machine-seamable base fabric and the second fabric ply are laminated to one another by needling staple fiber batt material therethrough. The press fabric is joined into endless form during installation on a paper machine by directing a pintle through the passage formed by the interdigitation of the seaming loops at the two widthwise edges.
The present invention is an alternative to those disclosed in these three patents in the form of a laminated multiaxial press fabric having more than one on-machine-seamable layer.
Accordingly, the present invention is an on-machine-seamable laminated multiaxial press fabric for the press section of a paper machine. The press fabric comprises a first base fabric and a second base fabric.
The first base fabric is a multiaxial base fabric having a first fabric ply and a second fabric ply fashioned from an endless base fabric layer. The endless base fabric layer comprises a fabric strip having a first lateral edge, a second lateral edge, a plurality of lengthwise yarns and a plurality of crosswise yarns. The fabric strip is spirally wound in a plurality of contiguous turns wherein the first lateral edge in a given turn of the fabric strip abuts the second lateral edge of an adjacent turn thereof, thereby forming a helically continuous seam separating adjacent turns of the fabric strip. The helically continuous seam is closed by attaching abutting first and second lateral edges of the fabric strip to one another, thereby providing the base fabric layer in the form of an endless loop having a machine direction, a cross-machine direction, an inner surface and an outer surface.
The endless base fabric layer is flattened to produce the first fabric ply and the second fabric ply having two widthwise edges. The first fabric ply and the second fabric ply are connected to one another at folds along the two widthwise edges. At least one crosswise yarn in each of the turns of the fabric strip are removed at each of the folds at the two widthwise edges to provide unbound sections of lengthwise yarns of said fabric strip at the folds, the unbound sections being seaming loops for joining the widthwise edges of the flattened base fabric layer to one another to form an endless loop.
The second base fabric is an on-machine-seamable base fabric of substantially the same length as the first base fabric and has a plurality of seaming loops along its two widthwise edges. The second base fabric may be one produced by a modified endless weaving technique. Alternatively, it may be a multiaxial base fabric like the first base fabric.
In either case, at least one layer of staple fiber batt material is needled into one of the first and second fabric plies of the first base fabric and through the second base fabric to laminate the first and second base fabrics to one another.
The seaming loops of the first base fabric and the seaming loops of the second base fabric join the first and second base fabrics into endless form in one of the following two manners.
Firstly, the seaming loops at opposite ends of the first base fabric and the seaming loops at opposite ends of the second base fabric interdigitate with one another to define a single passage through which a pintle is directed to join the press fabric into endless form.
Alternatively, the seaming loops at opposite ends of said first base fabric interdigitate with one another to define a first passage through which a first pintle is directed to form a first seam, and the seaming loop at opposite ends of the second base fabric interdigitate with one another to define a second passage through which a second pintle is directed to form a second seam. The first seam is on the opposite side of the second seam relative to the first base fabric.
The present invention will now be described in more complete detail with frequent reference being made to the figures identified below.